1. Field of the Invention
The invention is generally related to the area of optical communications. In particular, the invention is related to dispersionless multimode optical tap filters.
2. The Background of Related Art
To meet the ever-increasing demands for high bandwidth and more flexibility in modern communication networks, utilizing optical fiber networks capable of transmitting multiplexed channel signals is becoming increasingly popular. Many optical devices have been designed to meet the demands. Practically, these demands are arising from three areas: 1) video traffic over the Internet, such as Netflix and Youtube etc., 2) mobile access via smart phones including voice over IP (VoIP), 3) cloud storage and rich media files migrating via the Internet. All these demands seem to boost considerably the requirements on the service provider and the booming of various data centers. To meet the higher speed requirement, service providers and data centers also need more flexible network devices or equipment to support debugging and monitoring of the signals originated from them.
The availability of vertical cavity surface emitting laser (VCSEL) technology facilitates high speed parallel data links that can transmit 12 to 24 parallel channels, each channel supporting more than 10 gigabit/sec (Gbps). VCSEL has recently been demonstrated for bandwidth even higher than 25 Gbps and promises ro provide low cost parallel data links among computers, storage devices and servers to form an aggregated data center. As networked computers and storage devices grow larger and larger in functions and complexities, monitoring the network becomes an essential task to ensure the communication quality and data integrity.
In the past, a data center used a conventional fuse coupler for tap monitoring function. An optical tap is essentially a 1×2 device, where a portion of the input signal is branched out to a photo-detector that converts the tapped signal to serve as a monitored signal. A fused coupler can be made for either a single-mode or a multimode fiber application. U.S. Pat. No. 4,798,436 discloses one type of optical fused couplers. US Pat. Pub. No.: 2005/0201677 teaches a multimode optical fiber coupler. FIG. 1 duplicates FIG. 2 of US Pat. Pub. No.: 2005/0201677 to show the structure of an exemplary multimode optical fiber coupler.
In a typical data center today, where the cost effectiveness is always concerned, a good portion of data links are supported by the VCSEL based multimode fibers. Thus, multimode fiber taps are becoming increasingly important. As the data rate goes higher, the fused optical coupler will have more mode oriented insertion loss degradation, especially for multimode fused coupler with non-symmetry tapping/splitting ratio. The degradation can be several dB in bit-error-rate (BER) testing. One of the causes for this issue may be explained as shown in FIG. 2.
FIG. 2 shows that lower order modes by low frequency sine wave curves can travel faster than higher order mode signal (shown as high frequency sine wave curves). Thus, a signal that enters a multimode tap coupler with both lower and higher order modes at the same time will experience a travel time difference between these modes when they traverse the coupler (as illustrated by small vertical bars in FIG. 2). The time difference between these modes is known as modal dispersion or the arrival time spread of the signal from A to B.
The model dispersion is a very serious impairment in a communication link as it limits the data bandwidth. The modal dispersion depends on a tap ratio. Other than a 50/50 splitting ratio among two output ports, they are different for various tap ratios as shown in FIG. 3. In other words, for a fused fiber coupler with 30/70 splitting ratio, the dispersions obtained from the two output ports are different. For data rate under 8 Gbps, the modal dispersion from tapping based on a fused coupler may not be a serious limit. But higher bandwidth data links such as 10 Gbps and beyond would not ignore this modal dispersion issue.
In addition to suffering from splitting or tapping ratio dependent modal dispersion, signal tapping based on multimode fiber fused couplers also has a tap ratio dependent mode distribution problems among the two output ports. Therefore, for high speed multimode fiber data links (typically>8 Gbps), one cannot use a fused tap coupler to accurately perform monitoring function for dispersion and mode health. A fused tap coupler can only be used to monitor the optical signal power.
The article “Simplified theory of the multimode fiber coupler”, The Bell System Technical Journal, Vol. 56 No. 5, Page 729 describes that the root cause for mode dispersion and mode dependent loss issue is due to the complicated coupling coefficient, which previously people found that depending on mode order, coupling length and position etc. and the coupling coefficient can be described in the following equations:
                          C                              A            i                    ⁢                      B            i                                      =                                    C                                    B              i                        ⁢                          A              i                                                  =              γ        ⁢                                            2              ⁢              Δ                                α                ⁢                              u            2                                v            3                          ⁢                                                                                                              K                                          2                      ⁢                      l                                                        ⁡                                      (                                          w                      ⁢                                                                                          ⁢                                              d                        /                        a                                                              )                                                  ⁢                                  cos                  ⁡                                      (                                          2                      ⁢                      l                      ⁢                                                                                          ⁢                      α                                        )                                                              ±                                                K                  o                                ⁡                                  (                                      w                    ⁢                                                                                  ⁢                                          d                      /                      a                                                        )                                                                                                                    K                                  l                  -                  1                                            ⁡                              (                w                )                                      ⁢                                          K                                  l                  +                  1                                            ⁡                              (                w                )                                                          ,FIG. 4 duplicates the figure on page 729 of the article to show the effects. Those skilled in the art shall understand better after reviewing the article.
The present invention disclosure teaches a solution for monitoring optical power, dispersion and mode distribution in high speed multimode fiber communication. One embodiment of the present invention is a multimode optical tap filter that is dispersionless and tap ratio independent.